Tropical Storm HELENE

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TROPICAL STORM HELENE DISCUSSION NUMBER 12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082006
500 AM EDT FRI SEP 15 2006
HELENE DOES NOT LOOK TOO MUCH DIFFERENT THAN IT DID SIX HOURS
AGO...EXCEPT FOR THE DEEP CONVECTION BUBBLING IN SLIGHTLY DIFFERENT
PORTIONS OF THE CIRCULATION. PERHAPS SOME CONVECTION HAS RECENTLY
INCREASED A LITTLE NEAR THE LOW-LEVEL CENTER...BUT EVEN THAT SEEMS
TO BE ON THE DECREASE AGAIN. THE ADVISORY INTENSITY REMAINS 40 KT
BASED ON A BLEND OF THE 06Z DVORAK ESTIMATES. HELENE APPEARS TO BE
INGESTING SOME DRY AIR...AND IT MIGHT TAKE A WHILE FOR INNER CORE
CONVECTION TO BECOME PERSISTENT. THEREFORE...THE OFFICIAL
INTENSITY FORECAST IS SLIGHTLY BELOW ALL OF THE AVAILABLE GUIDANCE
AND IS ALSO A SLIGHT DOWNWARD ADJUSTMENT FROM THE PREVIOUS
ADVISORY. HELENE IS STILL FORECAST TO BECOME A HURRICANE IN A
COUPLE OF DAYS...JUST NOT QUITE AS STRONG AS THE SHIPS AND GFDL ARE
SUGGESTING.
HELENE CONTINUES WEST-NORTHWESTWARD AT ABOUT 13 KT ALONG A HEADING
BETWEEN 285 AND 290 DEGREES...WITH STEERING PROVIDED BY THE
SUBTROPICAL RIDGE TO ITS NORTH. THE DYNAMICAL MODELS ARE IN
REASONABLE AGREEMENT ON A WEST-NORTHWESTWARD TO NORTHWESTWARD
MOTION DURING THE NEXT 48 TO 72 HOURS. AFTER THAT THE MODELS
DIVERGE FAIRLY SIGNIFICANTLY...BOUNDED BY THE NOGAPS ON THE LEFT
AND TURNING THE CYCLONE WESTWARD...AND THE GFDL ON THE RIGHT AND
RACING HELENE NORTHWARD. THE DISAGREEMENT SEEMS TO STEM FROM HOW
EACH MODEL HANDLES A MID- TO UPPER-LEVEL TROUGH CURRENTLY OVER THE
EASTERN UNITED STATES THAT WILL BE MOVING SLOWLY EASTWARD DURING
THE NEXT SEVERAL DAYS. IF THE NOGAPS IS CORRECT...ENOUGH RIDGING
WILL REMAIN BETWEEN THAT TROUGH AND HELENE TO ALLOW THE TROPICAL
CYCLONE TO TURN WESTWARD. THE GFDL AND GFS...HOWEVER...FORECAST
HELENE TO GAIN ENOUGH LATITUDE BY ROUNDING THE RIDGE SUCH THAT THE
APPROACHING TROUGH WILL PICK UP HELENE. I AM NOT YET CONFIDENT THE
RIDGE TO THE NORTH OF HELENE WILL BREAK DOWN THAT FAST...SO THE NEW
OFFICIAL TRACK FORECAST LIES ON THE LEFT SIDE OF THE MODEL GUIDANCE
ENVELOPE...AND IT IS ESSENTIALLY IDENTICAL TO THE PREVIOUS
ADVISORY.
FORECAST POSITIONS AND MAX WINDS
INITIAL 15/0900Z 15.0N 40.3W 40 KT
12HR VT 15/1800Z 15.9N 42.1W 45 KT
24HR VT 16/0600Z 17.0N 44.2W 50 KT
36HR VT 16/1800Z 18.0N 46.0W 55 KT
48HR VT 17/0600Z 19.0N 47.4W 65 KT
72HR VT 18/0600Z 21.0N 50.0W 75 KT
96HR VT 19/0600Z 23.0N 52.5W 80 KT
120HR VT 20/0600Z 24.5N 54.5W 85 KT
$$
FORECASTER KNABB
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